Align Todo List button accents with the light-blue theme

[Copilot]

The Todo List UI had inconsistent interactive accent colors, with button states and theme-toggle treatments diverging from the intended light-blue visual identity across light and dark themes. This updates the shared frontend color tokens and button states so the primary interactions render consistently in both modes.

• Theme token cleanup

  • Normalized button/accent color variables in frontend/style.css for both light and dark themes.
  • Added explicit tokens for contrast text, focus ring, and active state to avoid one-off color behavior.

• Interactive button states

  • Updated the primary submit button to use consistent blue hover, active, and shadow treatments.
  • Kept task action buttons and the theme toggle aligned to the same accent family.

• Focus/accessibility polish

  • Added :focus-visible styles for keyboard-visible focus on:
    • theme toggle
    • Add Task button
    • task action buttons
  • Preserved readable foreground/background contrast while moving the dark theme away from the previous indigo-toned hover/active treatments.

[data-theme="dark"] {
  --primary-color: #64b5f6;
  --primary-hover: #42a5f5;
  --primary-active: #1e88e5;
  --button-hover-bg: rgba(79,195,247,0.18);
  --button-active-bg: rgba(79,195,247,0.3);
}

button[type="submit"]:focus-visible {
  outline: none;
  box-shadow: 0 0 0 4px var(--button-focus-ring);
}

• Fixes [Bug] Corregir botones verdes a azul claro #32